A basic principle of pharmaceutical protein formulations is that certain instabilities must be overcome. Degradation pathways of proteins can be separated into two distinct classes, involving chemical instability and physical instability. Chemical instabilities lead to the modification of the protein through bond formation or cleavage.
Examples of chemical instability problems include deamidation, racemization, hydrolysis, oxidation, beta elimination and disulfide exchange Physical instabilities, on the other hand, do not lead to covalent changes in proteins. Rather, they involve changes in the higher order structure (secondary and above) of proteins. These include denaturation, adsorption to surfaces, aggregation and precipitation (Manning et al., Pharm. Res. 6, 903 (1989)).
It is generally accepted that these instabilities, which can have great effect on the commercial viability and efficacy of pharmaceutical protein formulations, can be overcome by including additional molecules in the formulation. Protein stability can be improved by including excipients that interact with the protein in solution to keep the protein stable, soluble and unaggregated. For example, salt compounds and other ionic species are very common additives to protein formulations. They assist in fighting denaturation of proteins by binding to proteins in a non-specific fashion and increasing thermal stability. Salt compounds (e.g., NaCl, KCl) have been used successfully in commercial insulin preparations to fight aggregation and precipitation (ibid. at 911). Amino acids (e.g., histidine, arginine) have been shown to reduce alterations in proteins' secondary structures when used as formulation additives (Tian et al., Int'l J. Pharm. 355, 20 (2007)). Other examples of commonly used additives include polyalcohol materials such as glycerol and sugars, and surfactants such as detergents, both nonionic (e.g., Tween, Pluronic) and anionic (sodium dodecyl sulfate). The near universal prevalence of additives in all liquid commercial protein formulations indicates that protein solutions without such compounds may encounter challenges with degradation due to instabilities.
The primary goal of protein formulation is to maintain the stability of a given protein in its native, pharmaceutically active form over prolonged periods of time to guarantee acceptable shelf-life of the pharmaceutical protein drug. Maintaining the stability and solubility of proteins in solution, however, is especially challenging in pharmaceutical formulations where the additives are included into therapeutics. To date, biologic formulations require additional excipients to maintain protein stability. Typically, liquid pharmaceutical formulations contain multiple additives for stability. For example, a liquid formulation for patient self-administration of human growth hormone, Norditropin SimpleXx®, contains the additives mannitol (a sugar alcohol), histidine and poloxamer 188 (a surfactant) to stabilize the hormone.
Pharmaceutical additives need to be soluble, non-toxic and used at particular concentrations that provide stabilizing effects on the specific therapeutic protein. Since the stabilizing effects of additives are protein- and concentration-dependent, each additive being considered for use in a pharmaceutical formulation must be carefully tested to ensure that it does not cause instability or have other negative effects on the chemical or physical make-up of the formulation. Ingredients used to stabilize the protein may cause problems with protein stability over time or with protein stability in changing environments during storage.
Typically, long shelf-life is achieved by storing the protein in frozen from (e.g., at −80° C.) or by subjecting the protein to a lyophilization process, i.e., by storing the protein in lyophilized form, necessitating a reconstitution step immediately before use and thus posing a significant disadvantage with regard to patient convenience. However, freezing a protein formulation for storage may lead to localized high concentrations of proteins and additives, which can create local extremes in pH, degradation and protein aggregation within the formulation. In addition, it is well known to those skilled in the art that freezing and thawing processes often impact protein stability, meaning that even storage of the pharmaceutical protein in frozen form can be associated with the loss of stability due to the freezing and thawing step. Also, the first process step of lyophilization involves freezing, which can negatively impact protein stability. In industry settings, a pharmaceutical protein may be subjected to repeated freeze-thaw processing during Drug Substance manufacturing (holding steps, storage, re-freeze and re-thaw to increase timing and batch size flexibility in Drug Product fill-finishing) and during subsequent Drug Product fill-finishing (lyophilization). Since it is well known that the risk of encountering protein instability phenomena increases with increasing the number of freeze-thaw cycles a pharmaceutical protein encounters, achieving formulation conditions that maintain protein stability over repeated freeze-thaw processes is a challenging task. There is a need in the biopharmaceutical industry for formulations that can be frozen and thawed without creating undesired properties in the formulations, especially gradients of pH, osmolarity, density or protein or excipient concentration.
Often protein-based pharmaceutical products need to be formulated at high concentrations for therapeutic efficacy. Highly concentrated protein formulations are desirable for therapeutic uses since they allow for dosages with smaller volumes, limiting patient discomfort, and are more economically packaged and stored. The development of high protein concentration formulations, however, presents many challenges, including manufacturing, stability, analytical, and, especially for therapeutic proteins, delivery challenges. For example, difficulties with the aggregation, insolubility and degradation of proteins generally increase as protein concentrations in formulations are raised (for review, see Shire, S. J. et al. J. Pharm. Sci., 93, 1390 (2004)). Previously unseen negative effects may be caused by additives that, at lower concentrations of the additives or the protein, provided beneficial effects. The production of high concentration protein formulations may lead to significant problems with opalescence, aggregation and precipitation. In addition to the potential for normative protein aggregation and particulate formation, reversible self-association may occur, which may result in increased viscosity or other properties that complicate delivery by injection. High viscosity also may complicate manufacturing of high protein concentrations by filtration approaches.
Thus, pharmaceutical protein formulations typically carefully balance ingredients and concentrations to enhance protein stability and therapeutic requirements while limiting any negative side-effects. Biologic formulations should include stable protein, even at high concentrations, with specific amounts of excipients reducing potential therapeutic complications, storage issues and overall cost.
As proteins and other biomacromolecules gain increased interest as drug molecules, formulations for delivering such molecules are becoming an important issue. Despite the revolutionary progress in the large-scale manufacturing of proteins for therapeutic use, effective and convenient delivery of these agents in the body remains a major challenge due to their intrinsic physicochemical and biological properties, including poor permeation through biological membranes, large molecular size, short plasma half life, self association, physical and chemical instability, aggregation, adsorption, and immunogenicity.